Electron gun utilizing a strip transmission line to extract electrons from a cathode



March 14. 1967 .c. CROWLEY-M|LLING 3,309,557

ELECTRON GUN UTI ING A STRIP TRANSMISSION LINE T0 EXTRACT ELECTRON-SFROM A CATHODE Filed July 8, 1963 2 Sheets-Sheet 1.

v qn/VEN'ToK nucHAeL CRowpEY CROLoLEY-MlbLM/G 8 M QJE Filed July 8, 1965March M. 1967 M. c. CROWLEY-MILLING 3,309,557

ELECTRON GUN UTILIZING A STRIP TRANSMISSION LINE T0 EXTRACT ELECTRONSFROM A CATHODE 2 Sheets-Sheet 2 United States Patent 3,309,557 ELECTRONGUN UTILIZING A S TRANSMIS- SION LINE T0 EXTRACT ELECTRONS FROM ACATHODE Michael Crowley Crowley-Milling, Higher Disley, England,assignor to Associated Electrical Industries Limited, London, England, aBritish company Filed July 8, 1963, Ser. No. 293,304 Claims priority,application Great Britain, July 12, 1962, 26,882/ 62 9 Claims. (Cl. 315)This invention relates to electron guns and more particularly toelectron guns for producing high energy elec tron pulses of shortduration.

The invention has an important application in high energy electronaccelerators.

For many experiments in neuclear physics it is desirable to have veryshort pulses of electrons accelerated to high energies. The means ofproduction of short pulses of electrons down to a duration of about 1.0p.860. are well known. Typically, an electron gun for producing suchshort pulses has a cathode which may be a tungsten spiral direct emitteror an indirect emitter such as an oxide coated cathode or one of thewell known impregnated sintered metal cathodes. Immediately in front ofand closely spaced from the cathode is a grid normally held negativewith respect to the cathode. In front of the grid are beam formingelectrodes and beyond this the anode of the gun. When positive pulsessufiicient to overcome the negative bias are applied to the grid,electrons can flow from the cathode through the grid and the beamforming electrodes and through an aperture in the anode into theaccelerator. The main difficulty with the use of this type of gun forvery short pulses of electrons is that the capacitance between the gridand the cathode and between grid and beam forming electrodes and of theconnecting leads tends to distort the applied pulse. This limitationneed not be serious for pulses down to about 0.05 ,usec, but it is nowdesirable to produce pulses as short as 10 #560. Methods of overcomingthis limitation by making the gun of co-axial construction have beenproposed, but this leads to difiiculty with the connections to thecathode heater system etc.

It is therefore an object of the present invention to provide anelectron gun in which distortion of the control pulses is sufficientlyreduced to allow electron bursts in the 0.001 sec. duration range to beproduced.

According to the present invention, an electron gun comprises a cathode,an aperture anode and a strip transmission line disposed transverselybetween said anode and said cathode, the inner conductor of said linehaving an aperture in alignment with said anode aperture and saidcathode and carrying a control grid extending across the said aperture,and the outer conductor of said line being apertured in alignment withsaid anode aperture and said cathode to allow the passage of electronsbetween cathode and anode.

In a preferred embodiment of the invention that portion of the saidouter conductor defining the aperture remote from the cathode forms abeam electrode for .focusing electrons passing through said aperture andmay conveniently carry a screen grid extending across the said aperture.

Preferably, the cathode is suitably insulated from and mechanicallysupported by the apertured portion of the outer conductor adjacent saidcathode.

Advantageously, an electrical resistor of the same impedance as thetransmission line may be connected in a circuit extending between theinner and outer conductors of said line.

Alternatively one end of the transmission line may be short circuitedwith respect to current pulses.

The components of the gun may be said to be aligned when, in operationof the gun, electrons emitted from the cathode pass through the innerconductor aperture, the control grid located across this aperture, theouter conductor aperture whose walls form a beam electrode to focus theelectrons, the screen grid located across said outer conductor aperture,and through the anode aperture to provide an electron source forapparatus located beyond said anode.

In the specific embodiments described, the cathode is shown to beprojected through a first aperture in said outer conductor so as to beadjacent the grid electrode. Alternatively, the cathode may be locatedoutside said first aperture which then extends across the electron path.

The term strip transmission line as used in the s ecification, is to beinterpreted as a transmission line comprising a first electricalconductor which is a strip of which the width is appreciably greaterthan the thickness and a second electrical conductor which partially orcompletely surrounds the first conductor.

The electron gun would be located in a suitable vacuum system inaccordance with well known practice.

The cathode may be an indirectly heated sintered metal-type and be apart of a turret system of the type disclosed in British patentspecification No. 697,823

whereby the cathode assemblies are replaceable without disturbance ofthe vacuum system.

By way of example, the invention will now be described with reference tothe accompanping drawings, in which:

FIG. 1 is a diagrammatic longitudinal section of an electron gunaccording to one embodiment of the invention; 1

FIG. 2 is a cross section along the line IIII of FIG. 1;

FIG. 3 is a cross section along the line III-III of FIG. 1.

FIG. 4 illustrates graphically the formation of a control pulse for thegrid in another embodiment of the invention in which one end of thetransmission line is shortcircuited;

FIG. 5 is a longitudinal cross section of an electron gun according tothe invention showing the constructional details of the gun; and

FIG. 6 shows the general arrangement of the invention in conjunctionwith a pulse generator.

Referring to FIGS. 1, 2 and 3, the electron gun comprises essentially anindirectly heated cathode 1, an axially apertured anode 2 and a controlgrid 3 mounted on an apertured portion of the inner conductor 4 of astrip transmission line whose outer conductor is formed by an upperportion and a lower portion which together surround the inner conductor4. The upper wall 5 is apertured to provide a beam forming electrodewhilst the lower wall 6 serves as a support structure for the cathode 1.The inner and outer conductors are coupled together through a capacitor7 which enables a negative bias to be applied to the grid 3 relative tocathode 1. In operation, positive pulses are applied to grid 3 throughconductor 4 which pulses overcome negative grid bias and allowcorresponding bursts of electrons to be emitted from the cathode. Theimpedance of capacitor 7 is chosen to be negligible at the control pulsefrequency. In the embodiment shown, between the capacitor 7 and theconductor 4 there is connected a resistor 8 offering the same impedanceas the transmission line to the control pulses. If the grid-cathodespacing and the impedance of the line are correctly chosen a short pulseapplied to one end of the line can be transmitted without distortion tobe absorbed in the resistor 8 at the other end of the line. With such anarrangement, control pulses of about 10- ,usec. duration can betransmitted to the grid 3 without distortion and correspondingly shortbursts of electrons are emitted from the cathode.

In order to reduce the size of the control pulses applied to conductor 4(typically from several hundred volts to about fifty volts amplitude)the beam electrode 5 may be fitted with a screen grid 9 in alignmentwith the cathode control grid and anode. In this case the two portions 5and 6 of the outer conductor are separated from one another by aninsulating film 6a in the manner shown in the embodiment of FIGURE 5 toallow the screen-grid to be maintained at a suitable positive potentialwith respect to the cathode.

In an alternative embodiment to that shown in FIGS. 1-3, the resistance8 may be omitted from the arrangement so that the transmission line iseffectively short circuited at one end. With this arrangement, providethat the positive pulse incident on conductor 4 has a sharp frontedwaveform, it will be reflected with change of polarity at the shortcircuited end of the line and will introduce a pulse train on grid 3which is the resultant of the incident and reflected pulses. Thepositive portion of this train will exceed the grid bias to providepositive control pulses for the cathode during which correspondinglyshort bursts of electrons will be emitted. The formation of such acontrol pulse is shown in FIG. 4 which is a plot of pulse voltageagainst time at the grid 3. V represents a positive pulse transmitted bythe conductor 4 to the grid and V the reflection of that pulse from theshort circuited end of the transmission line. The resultant pulse trainat the grid is represented in FIG. 4 by the full line of which theportion V exceeding the grid bias magnitude V represents the controlpulse for the grid.

The duration of these control pulses are dependent only on the phaseshift between the incident and reflected pulses at the grid 3 and thisis in turn dependent on the length of transmission line between the gridand the short circuited end. Thus with such an arrangement the durationof the control pulse is largely independent of that of the incidentpulses so that resultant control pulses and electron bursts of 10-,uSfiC. duration or less can be easily produced using comparatively longincident pulses.

FIG. 5 shows the constructional details of an electron gun according tothe invention. A cathode 1 is mounted on a ceramic support plate 10supported by a structure 6 which forms the innermost portion of theouter conductor of a transmission line. The outermost portion of thisouter conductor is formed by cover plates 11 and a beam formingelectrode 5. The inner conductor 4 of the line has an apertured portionwhich supports a control grid 3. The additional capacitance introducedby the close spacing of the cathode and grid is oilset by cutting away aportion of the structure 6 at the position 13. The conductor 4 issupported near the grid by ceramic insulators 14 to maintain the correctgrid to cathode spacing.

An apertured anode 2 of the gun is supported by a mounting plate 17which may be the end plate of the accelerator vacuum chamber, and thegun base plate 16 is supported from this mounting plate by theconventional insulating means 18. The ends of the inner conductor 4 ofthe strip line are connected to glass-metal sealing terminals 12 wherethey pass through the end plate 16 of the gun. These seals are of suchconstruction as to maintain the correct surge impedance for the stripline. Additional glass-metal seals are used for the connection to thecathode 1.

It is to be understood that the invention is not limited to an electrongun including the particular constructions of strip transmission lineshown in the figures but that any strip transmission line comprising aninner conductor which is a strip of which the width is appreciablygreater than the thickness and an outer conductor which partially orcompletely surrounds the first conductor may be employed.

In FIG. ,6 a pulse generator 19 is connected to the gun by means of atransmission line 29 which may be a strip line of the samecharacteristics as that in the gun, or of different type ofcharacteristics it suitable matching arrangements are included. Thetransmission line may be interrupted, as far as direct current isconcerned, by capacitive elements 21 to prevent the grid-bias from beingapplied to the pulse generator. This grid-bias is applied to theconductor 4 carrying the grid 3 by means of a battery or other source 22through the inductor 23 or other form of high impedance to the pulsevoltage, connected to the central conductor of the transmission line,with suitable matching arrangements, not shown, to prevent mis-matchingof said transmission line. The system would be suitably evacuated and24- shows a part of the envelope.

What I claim is:

1. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path to provide for thepassage of electrons.

2. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path to provide for thepassage of electrons, and means for applying a control voltage to saidinner conductor to control the electron flow.

3. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path provided for thepassage of electrons, and grid wires extending across the aperture insaid inner conductor.

4. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path to provide for thepassage of electrons, grid wires extending across the aperture in saidinner conductor, and means for ap-- plying a control potential to saidgrid wires to control the electron flow.

5. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshav' ing apertures aligned with the said electron path to pro' vide 'forthe passage of electrons, grid wires extending across the aperture insaid inner conductor, a pulse gen" erator connected to one end of saidtransmission line, a' resistive load connected to the further end ofsaid transmission line, and means providing for the propagation ofpositive pulses from said pulse generator to said grid, said pulsesbeing thereafter absorbed by the resistive load.

6. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path to pro vide for thepassage of electrons, grid wires extending across the aperture in saidinner conductor, a pulse generator connected to one end of saidtransmission line, pulse reflecting means at the other end of saidtransmission line, and means for so propagating the voltage pulses alongsaid line from said pulse generator that the pulse train passes saidgrid and thereafter is returned to said generator by said pulsereflecting means to provide an electron beam that passes between theinitial incidence of the pulse train on said grid and the reflectedincidence thereof.

7. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said outer conductor havingopposed wall portions extending transversely of said electron path withone of said wall portions extending between said cathode and said anode,said inner conductor and said wall portions having apertures alignedwith the said electron path to provide for the passage of electrons,said cathode being positioned in the aperture formed in the other ofsaid wall portions.

8. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner conductor and an outer conductor, said inner and outer conductorshaving apertures aligned with the said electron path to provide for thepassage of electrons, means for biasing said inner conductor relativelyto said cathode, and a pulse generator capacitively coupled with an endof said inner conductor.

9. An electron gun comprising a cathode and an anode, a striptransmission line extending transversely across the electron pathbetween the cathode and anode, said strip transmission line including aninner strip conductor and an outer hollow conductor having an interiorspace of rectangular cross-section, with said inner strip conductorbeing disposed at least partially in said space, said inner and outerconductors having apertures aligned with the said electron path toprovide for the passage of electrons.

References Cited by the Examiner UNITED STATES PATENTS 3,237,047 2/1966Webster 3155.29

HERMAN KARL SAALBACH, Primary Examiner.

R. COHN, Assistant Examiner.

1. AN ELECTRON GUN COMPRISING A CATHODE AND AN ANODE, A STRIPTRANSMISSION LINE EXTENDING TRANSVERSELY ACROSS THE ELECTRON PATHBETWEEN THE CATHODE AND ANODE, SAID STRIP TRANSMISSION LINE INCLUDING ANINNER CONDUCTOR AND AN